The present invention relates to fuel burners and more particularly to an improved separator, to be used during low load operation of fuel burners in connection with pulverized coal-fired furnaces, which improves low load operation by separating the air entrained pulverized coal into a rich coal stream and a lean coal, moisture-ladened stream.
In view of today's fluctuating electricity demand, electric utilities have chosen to cycle many of their conventional coal-fired steam generator boilers by operating them at full load during peak demand hours and reducing them to low loads during periods of minimum demand. During these low load periods it is customary to burn auxiliary fuels in the boilers.
Auxiliary fuels burned in pulverized coal-fired utility boilers, principally diesel oil and natural gas, have become increasingly expensive while their availability has become uncertain. Government regulations are adding further pressures to reduce auxiliary fuel consumption utilities. However, auxiliary fuel consumption has been increasing because of cycling, low NOx burners and decreasing fuel quality.
Another problem associated with operating a coal-fired burner at low loads results in the fact that the pulverizing mills typically operate with a relatively constant air flow over all load ranges. When furnace load is reduced, the amount of coal pulverized in the mills decrease proportionally while the amount of primary air used to convey the pulverized coal from the mills through the admission assemblies into the furnace remains fairly constant, thereby causing the fuel-air ratio to decrease. When the load on the furnace is reduced to the low levels desired during minimum demand periods, the fuel-air ratio has decreased to the point where the pulverized coal-primary air mixture has become too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available. The coal/air mixture leaving the pulverizer not only has a large amount of excess air but also has a high moisture content. The high moisture laden air and high burner nozzle exit velocities result in poor combustion performance. In order to improve the combustion characteristics of this mixture, some of the excess moisture laden air is separated to create a coal rich and coal lean stream. The amount of air extracted by the separator depends on the type of coal used and the operating conditions.
Another final problem associated with operating a coal-fired burner at low loads is inherent in the properties of the coal to be burned. When burning coals with high surface moisture content such as lignite, it is necessary to dry the coal by adding excess hot air to the pulverizer to remove the moisture from the coal. High moisture content and a large volume of excess air result in poor combustion.
Typically separators have been of the cyclone separator type which are satisfactory for vertical burner applications, U.S. Pat. No. 2,293,737 (Hardgrove). However, most coals are fired horizontally. A horizontal application requires air for transporting the more dense fuel stream to the burner unlike the gravity feed of a vertical cyclone separator. Cyclone separators become impractical for horizontal discharge due to their size and pressure losses.
U.S. Pat. No. 4,274,343 (Kokkinos) discloses a separator used with horizontally fired pulverized coal. Centrifugal force separates the pulverized coal into a high coal concentration portion and a low coal concentration portion as the fuel passes through the elbow. A plate disposed along the longitudinal axis of the coal delivery pipe maintains the concentration of the two portions until each is discharged into the furnace through separate tiltable nozzles.